I've rubbished the quality of Morrison's photo paper several times on AB because it doesn't produce the colour quality that I'm seeking for my photos. However last Christmas I decided to see if the stuff that had lain unused in a cupboard for ages could actually be used for my Christmas cards. I found that the image quality was good enough for something that was destined to be thrown away anyway and (more importantly here) that I had no problems printing on the reverse side. So perhaps you might find it worth a try?
Prior to that I'd produced greetings cards by printing the inside on 100gsm paper and fixing the paper inside the card with a few spots of glue. I've also produced postcards by printing the reverse side on big sticky labels.